The present invention relates to a thermal transfer printing apparatus capable of performing multi-color printing by superposing plurality of colors. The present invention also is concerned with an ink paper cassette for use in this printing apparatus.
Terminal output units have been known which are capable of printing letters and patterns in accordance with various kinds of information output from devices such as a communication apparatus or computers. In recent years, there is an increasing demand for color printing in addition to monochromatic (black and white) printing. Among various printing techniques available for multicolor printing, a thermal transfer printing method is considered as being most promoting because of simple construction and maintenance-free operation, and this type of multi-color printing apparatus has been put to practical use.
Hitherto, a thermal transfer type printing system employs a thermal head which comprises a plurality of arrays of fine heating elements which are externally controllable. In operation, the thermal head is pressed onto an ink sheet composed of a sheet-like base and a heat fusible solid ink applied to the surface of the base so as to print information on a desired print paper.
Multi-color printing with an ink sheet of the type mentioned above is conducted by repeating printing operation a plurality of times with inks of different hues such that images of different hues or colors are superposed on one another. To this end, the ink sheet has regions of a predetermined length in which inks of different colors have been applied such that the regions of different colors appear sequentially and repeatedly along the length of an ink sheet. After completion of printing in one of the colors, printing is conducted with the ink of the next color which is provided in a subsequent region of the ink sheet, and this operation is repeated so that images of successive colors are superposed thereby attaining the multi-color printing.
Thus, the use of an ink sheet of the kind described in a thermal transfer printing apparatus essentially requires means for supplying the ink sheet and means for collecting used portion of the ink sheet.
Coloring materials ordinarily used in thermal transfer printers, such meltable pigments, as well as heat subliming dyes, can be used as the coloring agent to be applied to the ink sheet. The sensitivity of the sheet in terms of the relationship between the amount of heat applied by the thermal head and the thickness of the color of the print varies depending on factors such as the kind of the ink, thickness of application of the ink, thickness of the sheet base member, and so forth. Thus, different ink sheets exhibit different sensitivity characteristics. In addition, a sublimiting dye does not exhibits linear sensitivity characteristics: namely, the density of the color developed does not change linearly in relation to the heat applied by the thermal head. When this type of ink sheet is used, therefore, it is necessary to control the heat input in a non-linear manner in relation to the color density, in order to obtain a desired color density. This requires that information concerning the non-linear sensitivity characteristics, e.g., 60-byte information with 20-bytes for each of three colors, be input by a suitable input means. Storage of the ink characteristics in the printing apparatus cannot cope with a change in the characteristics of ink sheet such as improvement due to, for example, development of a novel dye. In order to obtain a desired printing performance, it is also necessary that various types of information concerning the ink sheet such as the number of colors carried by the ink sheet, length of each color region and so forth are input to the thermal transfer printing apparatus.
Hitherto, a module type ink sheet cassette of the type disclosed in Japanese Patent Unexamined Publication No. 56-67278 has been proposed for various reasons such as easiness of handling. This type of ink sheet cassette has an ink sheet supply shaft and an ink sheet takeup shaft accommodated in a cassette case and can easily be mounted on and demounted from the thermal transfer printing apparatus. Thus, the known ink sheet cassette has a supply shaft on which new or unused thermal transfer ink sheet is wound and a takeup shaft for taking up used ink sheet. These shafts are arranged rotatably at a predetermined spacing from each other with the ink sheet stretched therebetween. This ink sheet cassette is mounted as desired on the printing apparatus so as to perform printing. This type of ink sheet cassette is disclosed, for embodiment, in the specification of U.S. Pat. No. 4,901,090.
This cassette merely supports these two shafts rotatably and has no means for giving ink sheet information to the thermal transfer printing apparatus. Thus, the known thermal transfer printing apparatus and ink sheet have suffered from the following problems or shortcomings.
(1) It is impossible to automatically transmit information concerning the numbers of colors carried by the ink sheet such as monochromatic, bi-color, tri-color, tetra-color etc. and so forth. It is also impossible to automatically change the printing sequence in accordance with the printing sequence. Namely, a laborious manual switching operation has been necessary.
(2) It is impossible to automatically transmit information such as sensitivity of inks, kinds of inks (whether sublimiting dye or heat meltable pigment) and so forth to the thermal transfer printing apparatus. Thus, the thermal transfer printing apparatus could not cope with a change in the characteristics such as sensitivity of the ink caused by an improvement in the ink. This problem, as well as the above-mentioned problem of necessity of troublesome manual work, has not been fully recognized.
Furthermore, it is to be pointed out that no proposal has been made heretofore as to a measure for discriminating colors when an ink sheet of bi-color, tri-color or tetra-color-type ink sheet is used. Furthermore, no consideration has been made to provision of means for facilitating mounting of shafts in the cassette.
To sum up, it is necessary that an ink sheet cassette to be used on a thermal transfer printing apparatus has means for delivering ink sheet information to the thermal transfer printing apparatus.
Such ink sheet information delivering means has to meet the following requirements.
(1) The ink sheet cassette must enable the printing apparatus to print information even when the information to be recorded is comparatively complicated.
(2) The information should be recorded at a portion which is easy to read when the cassette is mounted on the apparatus.
(3) The ink sheet cassette must enable an easy alteration of the content of information, without requiring substantial modification of dies for forming the cassette.
(4) The cassette should be designated to avoid breakdown of the information due to mis-handling of the cassette.
When a specifically high degree of precision of scanning speed is required for reading of the ink sheet information recorded in the ink sheet cassette, it is necessary that a highly precise mechanism be employed specifically for the purpose of reading, with the result that the production cost is raised undesirably.
Various methods have been proposed and used for formulating ink sheet information into a bar code and to provide the bar code on the cassette. One of such methods is disclosed in the specification of U.S. Pat. No. 4,970,531.
It is possible to print a bar code on a label or the like and to manually adhere the label on an outer surface of the cassette case. In such a case, there is a risk that the record of the ink sheet information may be impaired due to touch of fingers. In another known method, the scanning for reading the recorded ink sheet information is conducted during insertion of the cassette into the printing apparatus. This method, however, essentially requires that the power supply has been turned on when the cassette is inserted. In addition, misreading tends to occur because the reading speed varies depending on the speed of inserting movement. The problems would be overcome if the scanning is conducted after completion of insertion. This however essentially requires a mechanism including a movable member. In addition, it is difficult to design and fabricate cassettes of a variety of colors. In a different method, a code registered section is provided on the leading end of the ink sheet. This arrangement eliminates any necessity for movable part for the scanning of the code because the tape itself runs during the printing. Unfortunately, however, the mark is readable only at the time of initial use of the ink sheet. It has also been attempted to put a mark representing the ink sheet information on a deckle edge of the sheet by adhesion or printing. This method, however, requires an impractically high degree of precision of the cutting, thus causing the production cost to rise.